This invention relates generally to centrifugal pumps, and, more particularly, to an improved method and apparatus for determining the operating point of a centrifugal pump.
As is known, a centrifugal pump has a wheel fitted with vanes and known as an impeller. The impeller imparts motion to the fluid which is directed through the pump. A centrifugal pump provides a relatively steady fluid flow. The pressure for achieving the required head is produced by centrifugal acceleration of the fluid in the rotating impeller. The fluid flows axially towards the impeller, is deflected by it and flows out through apertures between the vanes. Thus, the fluid undergoes a change in direction and is accelerated. This produces an increase in the pressure at the pump outlet. When leaving the impeller, the fluid may first pass through a ring of fixed vanes which surround the impeller and is commonly referred to as a diffuser. In this device, with gradually widening passages, the velocity of the liquid is reduced, its kinetic energy being converted into pressure energy. Of course it is noted that in some centrifugal pumps there is no diffuser and the fluid passes directly from the impeller to the volute. The volute is a gradual widening of the spiral casing of the pump. Centrifugal pumps are well known and are widely used in many different environments and applications.
The prior art also refers to centrifugal pumps as velocity machines because the pumping action requires first, the production of the liquid velocity; second, the conversion of the velocity head to a pressure head. The velocity is given by the rotating impeller, the conversion accomplished by diffusing guide vanes in the turbine type and in the volute case surrounding the impeller in the volute type pump. With a few exceptions, all single state pumps are normally of the volute type. Specific speed Ns of the centrifugal pump is NQ1/2/H3/4. Ordinarily, N is expressed in rotations per minute, Q in gallons per minute and head (H) in feet. The specific speed of an impeller is an index to its type. Impellers for high heads usually have low specific speeds, while those for low heads have high specific speeds. The specific speed is a valuable index in determining the maximum suction head that may be employed without the danger of cavitation or vibration, both of which adversely effect capacity and efficiency. Operating points of centrifugal pumps are extremely important.
Several common methods are employed in the prior art to determine the actual operating point of a centrifugal pump. Each of these methods is based on the basic premise that any two independent pump variables accurately measured will determine the operating point of a centrifugal pump. The operating point of a pump is commonly thought of as the flow rate and Total Dynamic Head (TDH) that the pump is delivering. The flow rate is sometimes referred to as a percentage of the Best Efficiency Point flow of that pump.
One method used to determine the operating point of a centrifugal pump is to physically measure the flow rate and TDH of the pump. Sensors are used to measure the pressure generated across the pump, flow, speed and temperature. The pressure generated across the pump can be measured using two transducers (one for suction pressure and one for discharge pressure) or one transducer (differential pressure transmitter across the pump). Speed and temperature measurements are required to make speed and specific gravity corrections to the pressure to calculate the TDH. Flow rate is the other principle measurement needed to plot TDH vs. flow. Flow rate can be measured using a variety of sensors from orifice plates to magnetic flow meters.
Another commonly employed method is to calculate the fixed speed motor""s electrical power. The two independent pump variables are Brake Horsepower (BHP) and speed. One way to calculate the electrical power is to measure the electrical current and calculate the kilowatt input to the motor. Once the kilowatt input is known the BHP output of the motor is calculated using motor efficiency data. Based on the hydraulic pump performance either typical for that model pump or the actual test data, for the actual speed of the pump, the operating point of the pump is determined from the intersection of the calculated BHP to the pump and the impeller diameter. This method requires only one sensor, an electrical current probe.
A similar but more accurate approach is to measure the total kilowatt input to the motor. This requires the measurement of two electrical currents and two voltages along with a kilowatt transmitter. This method will automatically correct for power factor. Again once the kilowatt input is known the BHP output of the motor is calculated based on motor efficiency. Referring to the hydraulic pump performance curve for the operating speed of the pump, the operating point can be determined.
Of the foregoing approaches, the first approach, namely, physically measuring the flow rate and TDH of the pump, is the most accurate means of determining the operating point of the pump, assuming proper use of instrumentation. However, this approach requires the purchase and installation of instruments to measure, suction pressure, discharge pressure, pumpage temperature, flow, and speed. Initial cost, installation and upkeep of all the sensors may not be justifiable.
The method of calculating the fixed speed motor""s electrical power also has several drawbacks. First, the electrical power factor is unknown and assumed to be 1.0. This is often not the case in actual plant installations. Second, actual pump performance may differ from the typical hydraulic data available for that model pump. Or, if the pump was actually tested, its performance in the plant may be different due to pumpage specific gravity or viscosity changes.
Measuring total kilowatt input into the motor eliminates one of the drawbacks of the previous method. That is, it eliminates the need to determine the electrical power factor to the motor. There still exists, however, error due to the discrepancy between the pump""s actual performance vis a vis typical or actual tested pump performance at the factory, due to specific gravity or viscosity changes.
A further drawback of the two latter methods is that the BHP curve on many centrifugal pumps varies little with changes in flow. A small error in BHP calculations will result in a large change in the operating point of the pump.
Improved methods for determining the actual operating point of a centrifugal pump are therefore desirable.
The invention provides a method and apparatus for determining the operating point of a centrifugal pump based on motor torque and motor speed. The method provides a way for determining whether the pump is operating within its normal flow operating range while eliminating the need for pump sensors employed using conventional methods.
According to one aspect of the invention, a method for determining whether a centrifugal pump is operating in a normal flow operating range is provided and includes the steps of: determining a motor torque/TDH relationship over a range of speeds for a minimum flow rate in order to obtain a minimum flow operating range for the centrifugal pump; determining a motor torque/TDH relationship over a range of speeds for a maximum flow rate in order to obtain a maximum flow operating range for the centrifugal pump; determining the actual operating motor torque and TDH of the centrifugal pump at a given operating point; and determining whether the actual operating motor torque and TDH of the centrifugal pump falls within the minimum flow and maximum flow operating ranges of the centrifugal pump.